1. Statement of the Invention
This invention relates to furan-modified phenolic resins which are potentially thermosetting for use in the coating of foundry sands.
2. Description of the Prior Art
The shell process for making metal castings and the like involves forming cores and/or molds from a sand coated with a thermosetting resin. In the practice of the shell process, a core or mold is formed in the desired configuration from the resin-coated sand, and then a metal is poured around the shell cores. The resin system slowly burns out, removing the resin binder from the system. As a result, the core collapses.
It has frequently been the practice to employ, as the resin in the shell process for making foundry core, phenolic novolak resins cured with hexamethylene tetramine. Such resins give high tensile strengths and consequently very strong cores.
One of the difficulties which has been incurred in the shell process stems from the incomplete decomposition or degradation of the resin binder. As will be appreciated by those skilled in the art, once the resin binder has been burned out or decomposes, the core, formed essentially of sand, becomes free flowing, and can be readily poured out of the casting. If, on the other hand, the phenolic novolak resin does not degrade to a sufficient extent, the core, or a portion thereof, remains inside the casting and must be removed by mechanical means. In some castings, such as engine blocks and heads, it is virtually impossible to remove pieces of core which have not been completely burned out and collapsed. Needless to say, if sand is left in an engine, a great deal of damage may result.
Iron and steel are generally poured at temperatures of the order of 2200 .degree.-3000.degree. F. At those temperatures, burnout of the phenolic novolak resins is usually complete. However, aluminum, brass, bronze as well as other metals and alloys having lower melting points, are poured at temperatures of the order of 1200.degree.-2000 .degree. F. At those lower temperatures, shakeout and collapsibility of cores is of greater concern. At such lower temperatures, phenolic novolak resins frequently do not burn out completely, with the result that cores or portions thereof are frequently left in the casting.